218 ZOOLOGY SECT. 



The Ctenophora are divisible into five orders as follows : 



ORDER 1. CYDIPPIDA. 



Ctenophora having two tentacles, retractile into sheaths, and 

 unbranched meridional and stomodseal vessels. The body is either 

 circular in section or is slightly compressed in the lateral plane 

 (Figs. 159 and 169). 



ORDER 2. LOB ATA. 



Ctenophora having numerous non-retractile lateral tentacles 

 contained in a groove : the bases of the two principal tentacles are 

 also present, but have no sheaths. The stomodseal and meridional 

 vessels unite with one another. The body is compressed in the 

 lateral plane, and is produced into two large oral lobes or lappets 

 and into four pointed processes or auricles (Fig. 170). 



ORDER 3. CESTIDA. 



Ctenophora having a band-like form, owing to the extreme 

 compression of the body in the sagittal plane. The bases of the 

 two principal tentacles are present, enclosed in sheaths, and there 

 are also numerous lateral tentacles contained in a groove. Union 

 or anastomosis of the meridional and stomodseal vessels takes 

 place (Fig. 171). 



ORDER 4. BEROIDA. 



Ctenophora having no tentacles. The mouth is very wide, and 

 the gullet occupies the greater part of the interior of the body. 

 The meridional vessels are produced into a complex system of 

 anastomosing branches (Fig. 172). 



ORDER 5. PLATYCTENEA. 



Flattened Ctenophora of creeping or sessile habit, with a pair of 

 retractile lateral tentacles. The costae (swimming-plates), when 

 present, are short and deeply sunk. There are no meridional 

 canals; but there is a system of branching peripheral vessels 

 (Figs. 173 and 174). 



Systematic Position of the Example. 



Hormiphora plumosa is a species of the genus Hormiphora, be- 

 longing to the family Pleurobrachiidw and to the order Cydippida. 



The presence of two tentacles, retractile into sheaths, and of 

 unbranched meridional canals places it in the order Cydippida. 

 In this order there are three families, amongst which the Pleuro- 

 brachiidce are distinguished by the absence of any compression of 

 the body, the transverse section being circular. The genus Hormi- 



